In the continuous casting of steel, in order to increase productivity, the flow of the continuous casting process must be performed continuously with as few interruptions as possible (that is, with a greater number of consecutive charges). Because most of the steel produced by continuous casting is aluminum-killed steel, a molten steel thereof contains a large amount of alumina produced by deoxidation, or reoxidation due to air or slag.
Consequently, when the casting time is lengthened by increasing the number of consecutive charges, adhesion of above-mentioned alumina and base metal tend to accumulate on the refractory pouring nozzle and cause nozzle blockages, which is one impediment in terms of increasing the number of continuous charges. As a countermeasure, conventionally, a method in which argon gas is blown into the molten steel inside the nozzle to achieve a cleaning effect, thereby preventing alumina buildup to the submerged nozzle has been widely used.
Furthermore, to prevent a reaction or adhesion occurring between the refractory materials and the molten steel or alumina or the like, the composition of the refractory materials of the nozzle has also been examined, leading to the development of a variety of adhesion resistant materials.
For example, Non-Patent Document 1 reports the results of investigating the alumina adhesion reducing effect achieved by applying a carbonless high-alumina refractory material to the submerged nozzle.
Furthermore, Non-Patent Document 2 reports that producing a low melting point compound in the ZrO2—C—CaO—SiO2 system is effective for preventing alumina adhesion.
On the other hand, to prevent the adhesion and solidification of base metal on the inside wall of the nozzle, keeping the nozzle at a high temperature has proved effective. Therefore, in the course of normal operation, the nozzle is sufficiently preheated by a gas burner or the like before beginning the casting process. Furthermore, a technique is known in which the nozzle is kept at a predetermined temperature by heating the nozzle during the casting process, thereby preventing the adhesion of base metal. Specific examples of this heating method include a method in which the nozzle itself generates heat, and a method in which heat is applied externally to the nozzle.
For example, as the above-mentioned method in which the nozzle itself generates heat, a technique is proposed in which a heating element is embedded inside the nozzle body, and the nozzle is heated by energizing the heating element (for example, refer to Patent Document 1).
Furthermore, a technique is proposed in which induction heating is performed using a nozzle in whose nozzle body is embedded a conductive refractory material with electrical resistivity of 102 Ω·cm (for example, refer to Patent Document 2).
On the other hand, as a method of heating the nozzle by supplying heat externally, a technique is proposed in which a block heater made of steel is disposed around the periphery of the nozzle (for example, refer to Patent Document 3). In this method, by using the block heater in combination with a sheath heater, the surface temperature of the nozzle can be raised to 850° C. or thereabouts.
Furthermore, as a high temperature heater, a carbon heater (carbon wire heating element) enclosed in a silica glass member is proposed (for example, refer to Patent Document 4). Moreover, as a preheating technique before casting begins, IH (induction heating) preheating can be used as an alternative to the typical gas burner preheating (for example, refer to Patent Document 5 and Patent Document 6). Because gas burner preheating requires time to preheat the nozzle, approximately 1.5 to 2 hours is needed from the start of preheating to the finish. On the other hand, because IH preheating has excellent heating efficiency, only 40 minutes or thereabouts is needed.
Generally, preheating of the nozzle is performed to prevent spalling due to thermal shock caused by the molten metal at the initial stage of casting, and to prevent the nozzle from becoming blocked when the molten metal loses sensible heat to the nozzle during casting, causing the formation of a solid layer of molten steel on the inside wall of the nozzle. In gas burner preheating, to improve preheating efficiency, and suppress a reduction in nozzle temperature in the interval after preheating before the nozzle is attached to the tundish, in recent years, the outer surface of the nozzle is sometimes covered by an insulating material.